mmg_233_2013_genetics_genomicswikiaorg-20200214-history
Plant Expansins in Bacteria and Fungi: Evolution by Horizontal Gene Transfer and Independent Domain Fusion
Lateral, or horizontal gene transfer (HGT) is nonsexual transmission of genetic material across species. This mechanism is common and important in prokaryotes, because it influences their function and evolution. However, in eukaryotes, HGT is relatively rare, but HGT in plants is relatively well documented. The transfer of genetic material between a host plant and its plant parasites, in most cases, requires direct physical association of the parasites and its host. Overview In this study, Nikolaidis and his colleagues investigated the relationship between plant expansin and EXLX1, which is a Bacillus subtilis protein that shares similar structure and function to plant expansins. They hypothesized that EXLX1 was acquired by bacteria through HGT from plants. Expansin and EXLX1 Expansins are nonenzymatic proteins found in plant cell wall. It consists of 250-275 amino acid residues and it is involved in cell wall loosening during development. Expansin contains two domians, an N-terminal domain (D1) with distant sequence similarity to family-45 endoglucanases catalytic domain and C-terminal domain (D2) which is related to a family of grass-pollen allergens. On the other hand, EXLX1 has very similar structure comparing to expansin (both D1 and D2 domains). Results Using BLASTp and tBLASTn (EXLX1 sequence as qurey), the authors identified several sequences from bacteria, fungi, and amoebozoa that align to both domians of EXLX1 (alignment E values ranged from 7e^-170 to 1e^-04 and scores ranged from 481 to 65.5), and they found high degree of homolgy between plant and nonplant expansins (EXLX1). Fold recognition program results show that these identified proteins indeed have both D1 and D2 domains like EXLX1 and expansin. Tertiary and primary sequence analyses show that EXLX1 has many homologs in phylogenetically diverged species. What is intriguing is that the tree derived from EXLX1 and its homolog in bacteria and fungi is different from the species tree. The best explanation for this phenomenon is that the evolution of these proteins is acient HGTs followed by verticle transimission in some instances. Parametric characteristics analysis supported this explanation, that no major difference were found across EXLX1 homologs in parametric characteristics. The next thing the authors looked into was when and which organismal lineage the expansin domain appeared. The first hypothesis is that the expansin domain have originated independently in plants, bacteria, and fungi and was then transmitted by HGT (independent fusion). However, since there was only homology rather than analogy among the expansin domain of bacteria, fungi amoebozoa, and plants, this hypothesis cannot stand. On the other hand, the other hypothesis is that expansin domain originated in bacteria or fungi or in the common ancestor of plants or algae and then vertically or horizontally transmitted to other species; and since vertical transmission would have led to massive gene losses (which is not the case), HGT must have played an important role in this process. Because of the fact that expansin exists in all plants, but only in a few bacterial and fungal species, the final conclusion is that expansin domain originated in plants and was transmitted to other species by HGT. In summary, in ''Bacillus subtilis, ''EXLX1 is a fusion protein of expansin domians that are transmitted through HGT from plants and homologous cellulose GH5 domains. Reference 1. Nikolaidis, N., et al. Molecular Biology and Evolution (2013) doi: 10.1093/molbev/mst206, Plant Expansins in Bacteria and Fungi: Evolution by Horizontal Gene Transfer and Independent Domain Fusion http://mbe.oxfordjournals.org.ezproxy.uvm.edu/content/early/2013/11/12/molbev.mst206.full.pdf+html 2. Expansin: http://www.answers.com/topic/expansin 3. Choi, D., et al. Physiologia Plantarum(2006) Volume 126, Issue 4, 511-518, Expansins: expanding importance in plant growth and development http://onlinelibrary.wiley.com/doi/10.1111/j.1399-3054.2006.00612.x/abstract